The present invention is concerned with novel apparatus and procedures for applying a coating onto a substrate or web. More particularly, this invention relates to methods and apparatus for guiding a coating composition stream emitted from a coating applicator onto a moving web and for altering the velocity of the stream emitted from the applicator.
It is well known in the art to coat a moving web with fluid discharged from an applicator spaced away from the web. For example, fluid may be so emitted from extrusion or curtain coaters such as those described in Coating Methods Survey of the Encyclopedia of Polymer Science and Technology, Vol. 3, pp. 777-782 by D. G. Higgens where the fluid free falls onto a web.
To control coating thickness, fluid velocity, contact angle of the coating fluid and web, etc., changes can be made in the pressure of the fluid in the applicator opening, the speed of the web, etc. But once the fluid has left the applicator, it is in free fall, i.e., free from guidance or restraint in falling, until it has reached the web. The only known apparatus for protecting the fluid stream from outside forces during deposition comprises enclosure sheets for deflecting stray air current. While it is well known to direct a gas current toward a coating bead, or to apply a vacuum to a coating head, (see, for example, U.S. Pat. Nos. 3,503,370; 3,640,752 and 3,676,178), such manipulations are not directed to the coating stream prior to contact with the web, and are particularly not directed toward a stream of coating composition in free fall. Furthermore, the forces provided by these techniques act normal to, rather than parallel to, the coating composition film or stream.
Manipulation of a coating composition stream is very desirable in many instances. For example, it might be desired to intermittently deflect the angle at which the stream hits a web or to adjust the stream velocity so as to achieve proper coating. It might also be advantageous to deflect the stream away from the web so that coating can be stopped without shutting down the applicator. In addition by applying a vibratory action to the air foils an induced chatter can be imparted to the coating which may be useful for certain applications.
While employment of the present invention brings to mind numerous applications, clearly the principle advantage obtained in a conventional curtain coating system is the ability to accelerate the falling stream to enhance impingement of the stream against a moving web to get better contact and thereby permit higher web speeds.